The vaccine-induced immune responses that, alone or in combination, will provide optimal protection for a large population of individuals against infection by HIV are still unknown. Nevertheless, many regard the induction of broadly neutralizing antibodies as one of the highest priorities of HIV vaccine research. The identification of monoclonal antibodies with broadly neutralizing properties (bnMAbs) is important to define neutralizing epitopes on the virus that should be targeted by an antibody-based vaccine, to facilitate the solution of a key structure in vaccine design, namely the functional envelope trimer to which only neutralizing antibodies bind with high affinity, and finally to help define conditions for antibody-mediated protection in animal models. Until very recently, only a handful of bnMAbs had been generated and this has undoubtedly restricted progress in immunogen development and structure determination. However recently the field has been energized as (a) potent and broad neutralization has been shown in the sera of a subset of infected individuals, (b) novel bnMAbs have been isolated that show considerably greater potency than the previously described bnMAbs but with comparable or better breadth (c) newer technologies successfully applied to novel bnMAb isolation are expected to yield many more such antibodies. The specific aims of the application are: 1. To use recently identified potent trimer-specific bnMAbs PG9 and PG16 to evaluate Env molecules in various forms as immunogens and as source material for structural determination. 2. To map the neutralization specificities in the sera of HIV-1 infected individuals with broad and potent neutralizing activity (including "elite neutralizers"). Initial progress on this aim has been very encouraging in suggesting novel epitopes recognized by multiple individuals. 3. To molecularly characterize novel broadly neutralizing human monoclonal antibodies isolated from donors investigated in Aim 2. The significance of this project is that it will increase our understanding of broadly neutralizing HIV epitopes, it will provide insight into how individuals naturally develop broadly neutralizing responses and it may lead directly to the design of immunogens able to elicit such responses. PUBLIC HEALTH RELEVANCE: An optimal HIV vaccine will likely elicit broadly neutralizing antibodies. This application seeks to understand broad neutralization at the molecular level to permit the design of such a vaccine.